1. Technical Field
Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing an image on a recording medium and an image forming apparatus incorporating the fixing device.
2. Description of the Background
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having two or more of copying, printing, scanning, facsimile, plotter, and other functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of a photoconductor; an optical writer emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a development device supplies toner to the electrostatic latent image formed on the photoconductor to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the photoconductor onto a recording medium or is indirectly transferred from the photoconductor onto a recording medium via an intermediate transfer belt; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
Such image forming apparatuses are requested to form a high quality toner image on a recording medium at high speed while saving energy. In order to address those requests, the image forming apparatuses employ fixing devices of various types, such as a roller type, a belt type, and a film type, configured to improve heating efficiency for heating the recording medium.
For example, the roller type fixing device may include a fixing roller heated by a heater and a pressing roller pressed against the fixing roller. As a recording medium bearing a toner image is conveyed through a fixing nip formed between the fixing roller and the pressing roller, the fixing roller and the pressing roller apply heat and pressure to the recording medium, melting and fixing the toner image on the recording medium.
The belt type fixing device may include a fixing belt looped over at least two rollers and a pressing roller pressed against the fixing belt to form a fixing nip between the pressing roller and the fixing belt. The at least two rollers may be a fixing roller pressing the fixing belt against the pressing roller and having a decreased thermal conductivity and a heating roller accommodating a heater.
The film type fixing device may include a thin, endless fixing film having a decreased heat capacity and a pressing roller contacting the fixing film to form a fixing nip therebetween. At the fixing nip, the pressing roller presses the recording medium against a heater via the fixing film. As the fixing film sliding over the heater conveys the recording medium through the fixing nip, the fixing film heated by the heater heats the recording medium.
Those fixing devices use a halogen heater, a ceramic heater, an induction heater, or the like as a heater that heats the fixing roller, the fixing belt, and the fixing film.
For example, JP-2007-079142-A discloses a fixing device incorporating an induction heater. A pressing roller is pressed against a fixing roller via a fixing sleeve to form a fixing nip between the pressing roller and the fixing sleeve. The fixing sleeve constructed of a release layer, an elastic layer, and a heat generation layer accommodates a fixing roller constructed of an elastic layer and a core metal. As a coil of the induction heater generates a magnetic flux that induces an eddy current in the heat generation layer of the fixing sleeve, the eddy current generates Joule heat that heats the recording medium conveyed over the fixing sleeve.
In order to address the request to form a high quality toner image, the temperature of the pressing roller may be controlled. For example, JP-2011-048167-A discloses a cooler situated outside the fixing device and configured to cool the fixing device.
Further, in order to address the request to form a high quality toner image at high speed while saving energy, the fixing roller, the pressing roller, and the heating roller may have a decreased heat capacity. However, the rollers having the decreased heat capacity may decrease the thermal conductivity in an axial direction of the rollers. Accordingly, as recording media of various sizes are conveyed over the rollers, the temperature of the rollers may vary in the axial direction thereof. For example, after a plurality of small recording media is conveyed over the rollers continuously, both lateral ends of the rollers in the axial direction thereof where the small recording media are not conveyed and therefore do not draw heat from the rollers may overheat. Consequently, uneven temperature of the rollers in the axial direction thereof may degrade the quality of the toner image fixed on the recording medium.